It is a conventional technique to use plural magnetic layers in order to improve the electromagnetic properties of magnetic recording media. The plural magnetic layers make it possible to differentiate the coercive force, the residual magnetic flux density and the magnetic particle size (represented by the crystallite size or the specific surface area) of the upper magnetic layer from those of the lower magnetic layer. The electromagnetic properties can therefore be enhanced by an improvement in outputs over a wide frequency range.
In recent years, audio cassette tapes have also under gone remarkable development with one object being to increase the densification thereof. Also, many proposals have been advanced with objects being to equalize the outputs over the wide frequency range by using the plural magnetic layers as described above and aiming at attaining higher sound quality. For example, these proposals are disclosed in JP-B-37-2218 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-40-5351, JP-B-54-8286, JP-B-52-28364, JP-B-58-100, JP-A-58-56230 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and U.S. Pat. No. 3,761,311.
Recently, 160% bias high position audio cassette tapes have attained commercial acceptance along side the conventional 100% bias normal position tapes. Examples of the former are disclosed in JP-A-58-56228, JP-A-54-21304, JP-A-54-48504, JP-A-57-33435 and U.S. Pat. No. 4,109,046. In these references, tape features are described such as the coercive force, the magnetic particle specific surface area and the residual magnetic flux density of a first magnetic layer and a second magnetic layer. Also, many descriptions therein are directed to the enlargement of the dynamic range of high and low tones. These references also include an improvement of a so-called slump in a middle tone range. However, it has become clear that these manipulations alone are insufficient to achieve high sound quality and are utterly insufficient in MOL characteristic at 10 kHz for so-called 3-head decks.
Specifically, JP-A-54-48504 proposes a magnetic recording medium comprising a second magnetic layer having a coercive force of 590 to 800 Oe and a residual magnetic flux density of 1,200 gausses or more, and a first layer having a coercive force of 400 to 560 Oe, but the residual magnetic flux density is not described for the first layer.
U.S. Pat. No. 4,423,453 further proposes a magnetic recording medium comprising a second magnetic layer having a coercive force of 590 to 800 Oe and a thickness of 2.1 .mu.m or more, and a first magnetic layer having a coercive force of 400 to 590 Oe and a thickness of 2.1 to 2.8 .mu.m.
In the above-described prior art approaches, the coercive force of the upper and lower layers could be differentiated only to a limited degree to improve a drop, the so-called slump, in frequency characteristics in the middle tone range, which causes the restriction on the enlargement of the dynamic range of high and low tones. Further, in a so-called 3-head recording deck in which recording heads are independently arranged, it was discovered that the MOL characteristic at 10 kHz was dramatically reduced with decreasing thickness of the second layer. This phenomenon is shown in FIG. 5. In a 2-head recording deck, the tendency of the MOL characteristic at 10 kHz to be reduced was not observed. However, in the 3-head recording deck, it was ascertained that the thin second magnetic layer reduced the MOL characteristic at 10 kHz, which resulted in deterioration in sound quality. The present inventors have conducted intensive studies in an effort to solve the problems of slump in the middle tone range and reduction in MOL characteristic at 10 kHz.